Adds support for pausing recordings.  When settings are eligible for
recordings, a pause button will appear next to the recording button.  If
the settings are not eligible, it will warn the user in the output
settings that they cannot pause recordings if those settings are used.

Code submissions have continually suffered from formatting
inconsistencies that constantly have to be addressed.  Using
clang-format simplifies this by making code formatting more consistent,
and allows automation of the code formatting so that maintainers can
focus more on the code itself instead of code formatting.

Apparently certain versions of Qt have a bug where the tray icon's
internal "visible" variable is set to false despite the fact that it
still shows.  Calling show() first before calling hide() sets the
internal boolean value and fixes the issue with the hide call.

Fixes "Toggle Preview" hotkeys from duplicating when profiles are
switched.  The hotkeys were not being unregistered.

(This commit also modifies the UI, obs-ffmpeg, and obs-output modules)

Fixes a long-time regression where the program would lock up if an
encode call fails.  Shuts down all outputs associated with the failing
encoder and displays an error message to the user.

Ideally, it would be best if a more detailed error could be displayed to
the user about the nature of the error, though the primary problem is
the encoder errors are typically not something the user would be able to
understand.  The current message is a bit of a generic error message;
improvement is welcome.

Another suggestion is to try to have the encoder restart seamlessly,
though it would take a significant amount of work to be able to make it
do something like that properly, and it sort of assumes that encoder
failures are sporadic, which may not necessarily be the case with some
hardware encoders on some systems.  It may be better just to use another
encoder in that case.  For now, seamless restart is ruled out.

This commit adds a preview to the properties window for transitions.

The preview will play back the transition at the global transition
duration or the transitions fixed duration, between two private scenes
with an A and B label, and different background colors.

This fixes Xcode warning about detecting UI usage from another thread.

The workarounds were made because of conflicts with running multiple UI
threads at once on macOS, which macOS can't do very well, and would be
susceptible to crashes.  This would cause crashes not only on startup
but seemingly at random when using the browser source on macOS.  The
original "fix" was a hack to try to minimize UI code and browser UI code
from executing at the same time.  The macOS initial scene loading was
deferred until all Qt-related and main window initialization was
completed.  Although this worked to some extent to prevent conflicts, it
made it so that there was an initial period on startup where the entire
UI seemed "blank" for users, and it was still possible for the main UI
thread and the browser UI thread to clash, causing crashes seemingly at
random for users.

The external message pump method of CEF is the solution to the problem,
which is the method which allows the main UI thread to share events with
CEF.  To do this, all CEF operations need to be performed in the UI
thread (Qt's main thread), and CefDoMessageLoopWork() needs to be called
when CefApp::OnScheduleMessagePumpWork callback is triggered.  A number
of other issues had to be solved as well, such as CefBrowser references
getting "stuck" in the Qt event queue.

With this, macOS no longer needs to do the "deferred load" hack, and
browsers are now much more stable and no longer as susceptible to
seemingly random crashes, improving overall program stability when
browsers are used.

The "What's New" dialog currently is only functional on windows.

Fixes a potential crash that can happen if the system tray is not
initialized for whatever reason.

Fixes https://obsproject.com/mantis/view.php?id=1443

Add D3D/GL debug markers to make RenderDoc captures easier to tranverse.

Also add obs_source_get_name_no_null() to avoid boilerplate for safe
string formatting.

Closes obsproject/obs-studio#1799

This solves the issue of users who close the primary dock widgets and
can't figure out how to get them back, who for whatever reason choose
not to read the message box when closing dock widgets that tells them
how to get it back.  The users can still hide the primary dock widgets
via the View menu, but they can no longer close them via the [x] button
on the dock widget.  This change does not apply to secondary dock
widgets, such as a chat window, stats, etc.

Closes obsproject/obs-studio#1804

This also modifies libobs.

This only happened on Linux.

Fixes https://obsproject.com/mantis/view.php?id=1018

Fixes a crash that can happen if you try to use the settings window
while in an even subloop, or if you try to close OBS while in an event
subloop.  Continually retries (defers) the actions every one second
until the subloop has finished.

Closes obsproject/obs-studio#1768

Originally added for completeness but wasn't actually implemented
correctly. Can be added back later if desired.

This new scale filter computes pixels by weighing the coverage area of
source pixels over the target pixel. This algorithm works well for both
upsampling and downsampling, but was mainly designed to upscale
high-quality low-resolution sources like RGB/HDMI retro consoles. I've
heard of people using odd workarounds like scaling up to very high
resolutions before scaling back down to preserve pixel shartpness. This
algorithm directly addresses this use-case in a much more direct
fashion.

The Area scale filter does a better job of preserving the thickness of
thin features than the Point filter.

The Area scale filter does not look at source pixels that lie outside
of the target pixel, leading to a much sharper image than Bilinear,
Bicubic, and Lanczos filters.

This filter should interpolate pixels in linear space, but OBS is not
equipped to do that at the moment.

libobs: Add GPU effect, and wire up scene serialization.

obs-filters: Add Area as an option for scale_filter.

UI: Add Area as an option for both scene items, and canvas downscaling.

Users don't realize that dockable windows can be closed (hidden) and can
be shown again via the View menu.  This adds an explicit warning when
the user first closes a dockable window for their first time.  In future
versions, this should be changed to a dialog box with a "Do not show
again" checkbox.

The Auto-Configuration dialog was being created outside of the Qt
message loop, which could cause a lockup with auth code.

Fixes an issue when Start Replay Button stays at checked state if stop
signal generated (hotkey event for example) instead of mouse click.

Mantis: https://obsproject.com/mantis/view.php?id=1343